1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly it relates to an image forming apparatus wherein a prevention means for preventing developer from shifting toward a recording medium is arranged in the proximity of recording electrodes.
2. Related Background Art
In the past, various image forming apparatuses capable of forming an image in response to image information have been proposed. Among them, there is an apparatus wherein an image is formed on a recording medium by electrostatically adhering conductive magnetic toner (fine powder developer) to the recording medium.
For example, Japanese Patent Laid-Open No. 51-46707 (corresponding to U.S. Pat. No. 3,914,771) discloses such technique. As shown in FIG. 10, conductive magnetic toner 51 disposed around a non-magnetic cylinder 50 is attracted onto an outer surface of the non-magnetic cylinder 50 by alternate magnetic field generated by a rotary magnet 52 arranged in a coaxial relation to the non-magnetic cylinder 50 and is conveyed along the outer surface of the non-magnetic cylinder. The toner 51 is conveyed to pass over recording electrodes 53 closedly spaced apart from each other and arranged on the outer surface of the non-magnetic cylinder along a longitudinal axis thereof. When the toner is contacted by a sheet-shaped recording medium 54 disposed in the vicinity of the non-magnetic cylinder 50 and comprising an inner conductive layer 54b and an outer insulation layer (or dielectric layer) 54a, a voltage is applied by an electric power source 55. By applying the voltage between the recording electrodes 53 and the conductive layer 54b of the recording medium 54 in response to image information, an image is formed on the recording medium by adhering the toner 51 to the insulation layer 54a of the recording medium 54.
In an image forming apparatus using the above-mentioned principle, as shown in FIG. 11, with respect to a recording medium 54 shifted by a drive feed roller 56a and a driven feed roller 56b, toner 51 is conveyed onto recording electrodes 53 by rotating a rotary magnet 52 disposed in a coaxial relation to a non-magnetic cylinder 50 arranged in a developing device 57. And, by selectively adhering and non-adhering the toner to the recording medium by selectively applying the voltage from a record controlling portion 58 to the recording electrodes, an image is formed on the recording medium.
For example, when the voltage of 40 V from the record controlling portion 58 is applied to the recording electrodes, the toner 51 is adhered to the recording medium 54; whereas, when the voltage is 0 V, the toner is not adhered to the recording medium. By alternating such operations, the image can be formed.
The toner 51 adhered to the recording medium 54 is displayed at an image display area 59 as a toner image. Then, the charges on the toner image are removed in an earthing direction by frictionally sweeping a surface of the recording medium by means of a cleaning member 60 comprising conductive carbon fibers, conductive resin, conductive rubber or similar material, with the result that the toner is dropped from the surface of the recording medium onto the non-magnetic cylinder 50 to be re-used. Incidentally, the residual charges remaining on the recording medium 54 are removed in an earthing direction by means of a charge removing brush 61.
With the above-mentioned arrangement, since the developer is the conductive magnetic toner, a toner brush (toner chains) is formed between the recording electrodes 53 and the recording medium 54 along the lines of magnetic force. Since the toner chain has low electric resistance, when the electric charges are applied from the recording electrodes 53 to the toner chains, the latter can contribute to the recording operation.
However, with the above-mentioned arrangement, since the length of the toner chain and the binding force between the toner particles (the stronger such binding force the smaller the electric resistance to facilitate the recording) depend upon the magnetic force of the rotary magnet 52, the following problems arise. That is to say, if the magnetic force of the rotary magnet 52 is weaker, the toner chain becomes shorter, with the result that, since it is difficult to obtain contact between the toner chains and the recording medium, it is feared that the recording is imperfect. On the other hand, if the magnetic force becomes stronger, the productivity of the rotary magnet 52 is decreased, thus making the apparatus expensive. Further, when the magnet is made of resin or rubber, the maximum magnetic flux density immediately above the surface of the magnet becomes 1000.about.1200 Gauss. In this case, since the image can be formed on the recording medium by contacting the toner chains on the recording electrodes 53 with the recording medium 54 only when a distance between the recording electrodes 53 and the recording medium 54 is 400 .mu.m at most, if the apparatus is large-sized, it is difficult to maintain the distance between the recording electrodes 53 and the recording medium 54 to 400 .mu.m due to the discrepancy in the manufacturing accuracy of parts of the apparatus.
For example, due to the imperfect straightness of the cylinder 50 shown in FIG. 11, imperfect straightness of the recording electrodes 53, and the distortion of the drive and driven feed rollers 56a, 56b, it is feared that the recording electrodes 53 contact with the recording medium 54 to damage them.